1. Field of the Invention
The present invention is directed to scanning probe microscopes, and more particularly, a scanning electrochemical potential microscope (SEPM) for characterizing a sample placed in a liquid by measuring an electrochemical potential that varies across an electrical double layer at the liquid/solid interface.
2. Description of Related Art
Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically use a sharp tip and low forces to characterize the surface of a sample down to atomic dimensions. Generally, SPMs include a probe having a tip that is introduced to a surface of a sample to detect changes in the characteristics of a sample. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular region of the sample and a corresponding map of the sample can be generated.
In an AFM, for example, in a mode of operation called contact mode, the microscope typically scans the tip, while keeping the force of the tip on the surface of the sample generally constant. This is accomplished by moving either the sample or the probe assembly up and down relatively perpendicularly to the surface of the sample in response to a deflection of the cantilever of the probe assembly as it is scanned across the surface. In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography. Similarly, in another preferred mode of AFM operation, known as TappingMode™ (TappingMode™ is a trademark owned by the present assignee), the tip is oscillated at or near a resonant frequency of the associated cantilever of the probe. The amplitude or phase of this oscillation is kept constant during scanning using feedback signals, which are generated in response to tip-sample interaction. As in contact mode, these feedback signals are then collected, stored and used as data to characterize the sample. Note that “SPM” and the acronyms for the specific types of SPMs, may be used herein to refer to either the microscope apparatus, or the associated technique, e.g., “atomic force microscopy.”
Another type of SPM is the scanning tunneling microscope (STM) a portion of which is shown in an exploded view in FIG. 1A for a tip-sample separation “S.” In an STM 10, similar to AFM, a probe 12 having a tip 14 is employed to scan a surface 19 of a sample 18. However, in STM, tip 14 is conducting. In addition, tip 14 is positioned an atomic distance, such as two to three atoms (i.e., approximately ten angstroms), above a surface 19 of sample 18. To reduce the area exposed to liquid, and thus the faraday (parasitic) current, an insulating layer 16 may be disposed on tip 14 so as to leave only the apex of the tip exposed to the liquid. Notably, when working in air, the insulating layer 16 is not required. And, because device operation is based on current flow between the probe tip and sample surface, the sample is typically a conductor or semiconductor.
In operation, a current, known as the tunneling current, is made to flow between sample 18 and the free end or apex of tip 14. This tunneling current is produced in response to a bias voltage applied between the sample 18 and tip 14 and is sensitive to the tip-sample separation distance “S.” In particular, for example, the current may increase by about a factor of ten in response to a one atom (approximately 2–3 angstroms) decrease in separation distance.
In an STM, the tip is typically fast scanned over the surface of the sample while the vertical position of the tip is monitored via the measured amount of current and, in response, feedback signals are generated to maintain the tip-sample separation generally constant. The tip is preferably coupled to a piezoelectric device that responds to positive and negative voltages generated based on the feedback signals to expand or contract, and thereby lower or raise the tip/sample relative to the other of the tip/sample. In sum, during operation, maintaining a constant tunneling current through the use of the feedback loop thereby gives a generally constant separation of the tip above the surface. Similar to AFM, these feedback signals are indicative of a particular characteristic of the sample.
In yet another type of SPM, similar to STM, a scanning electrochemical microscope (SECM) utilizes a technique in which a current flows through a small electrode tip near a conductive, semiconductive or insulating sample immersed in a solution, as shown and described in commonly owned U.S. Pat. No. 5,202,004. SECM can be used to characterize processes and structural features of the sample surface as a tip is scanned near the surface. In general, SECM can provide surface topography information and analytical data at greater tip-sample separations. Further, since SECM operation is based on electrochemical reactions, microfabrication can be carried out with this apparatus. For example, etching and deposition of metals and semi-conductors and synthesis (for example, electropolymerization) are possible.
As shown in FIG. 1B, an SECM 20 includes a tip 22 having an insulative coating 26 that leaves exposed an uncoated apex 24 of tip 22 to facilitate electrochemical reactions during operation, as described below. Notably, the ultimate resolution of the SECM depends primarily upon the tip size and shape. In addition, the solution resistance and mass and charge transfer process rates that effect the current density distribution are also important performance factors.
While SECM apparatus resembles the STM, there are fundamental differences between the two in both principle of operation and range of applications. In SECM, again as shown in FIG. 1B, the current is carried by reduction-oxidation (hereinafter “redox”) processes at the free end 24 of tip 22 and sample 18, and is controlled by electron transfer kinetics at the interfaces and mass transfer processes in solution. As a result, SECM 20 can make measurements at large tip-sample spacings (for example, in the range of 1 nanometer to 10 μm).
To the contrary, because STM depends on the flow of a tunneling current between the tip and sample, the distance between the two typically must be on the order of 1 nanometer or less, as noted above, and surface topographic x-y resolution of this size scale is usual. In addition, even for STM applications in solution, the tunneling current is non-faradaic (i.e., no chemical changes in solution components or sample surface species occur due to the tunneling current), so that unlike SECM, the tip current cannot be related directly to the sample potential.
On the other hand, the tip 12 used by STM 10 is sharp, thus allowing measurements to be made with high resolution. Again, in SECM, to facilitate the aforementioned redox processes, tip 22, and more particularly free end or apex 24 is more blunt than STM tip 14, which correspondingly compromises SECM imaging resolution.
In addition to the above, both STM and SECM have other significant drawbacks for electrochemical applications. In electrochemical STM, the highly concentrated solution compromises image resolution by interfering with the tunneling current. Moreover, for biological applications, because the tip-substrate separation is extremely small (less than 1 nanometer, as mentioned above), if the biological object is relatively large, the tip may penetrate into the object being scanned, thus clearly compromising the integrity of the image. This is exacerbated by the fact that biological molecules are poor conductors, thus providing no relief with respect to the narrow tip-sample separations. For example, if one places a DNA molecule on top of a conducting substrate and tries to perform STM to characterize the DNA molecule, it will be extremely difficult to generate sufficient current to pass through the molecule. Thus, some sample measurements are impossible with an STM. Further, even if the sample is an adequate conductor, the STM is limited by the fact that, because it does not operate based on force feedback, the user is often unaware of how much force the STM tip is exerting on the sample, thus leading to damaged samples.
When employing SECM, as mentioned previously, the preferred working distance is much greater to accommodate the electrochemical reaction. In any event, however, the working distance “S” (FIG. 1B) is preferably at least 100 nm to achieve adequate performance. Larger tip-sample working distances yield very poor resolution and thus, in essence, the preferred SECM working distance is too great. A typical SECM has a working distance range that may extend hundreds of micrometers. This is primarily due to the fact that the tip must be rather large (compared to, for example, an STM tip), so as to achieve the necessary redox effects. Again, compared to STM tip 14 shown in FIG. 1A, the SECM tip 24 shown in FIG. 1B is relatively dull and resolution is correspondingly compromised.
Moreover, with further reference to SECM, the system is performing local chemistry via the electrochemical reaction. Although this aspect of the system affords SECM the ability to perform a wide range of applications such as etching, etc., a major drawback in this regard is that it is difficult to maintain reliable operability of the electrodes for extended periods of time.
In an alternative to SECM and STM, measurements across an electrical double layer that exists near the surface of a sample immersed in a liquid can be made. Generally, when the sample is placed in an ionic solution, the solution has ions which align along the sample surface. Moreover, the charged surface is characterized by a degree of ordering of the ions some distance away from the sample surface. If the sample surface is negatively charged, a layer of positive ions in the liquid align along the sample surface, then a layer of negative ions in the liquid generally align on top of the positive ions, and so forth, for a particular distance from the sample surface.
The electrical double layer phenomenon has been known and analyzed. One known system includes disposing a quartz ball on the end of a cantilever, and then applying an electrostatic force on the ball to measure sample variations in the double layer. Clearly such a system operates on a macroscopic scale. The quartz ball is several microns in diameter, and thus the resolution is correspondingly poor, and certainly insufficient for the applications contemplated by the present invention. Another known system using a method called scanning vibrating electrode technique (SVET) measures and maps the electric fields to characterize localized electrochemical activities. A differential electrometer is employed in conjunction with a lock-in amplifier to perform a.c. detection so as to improve sensitivity. This system, for example, could be used to measure localized corrosion events. However, the spatial resolution is only about a few tens of micrometers, again unacceptable for the presently contemplated applications.
Using the above types of metrology tools, a number of different applications in which electrochemistry is desirably monitored include electroplating in integrated circuit fabrication. In particular, it is desirable to monitor the crystallization of the plating because crystallization is indicative of the integrity of the plating, including conducting characteristics, etc., as appreciated by those skilled in the art. Another application of particular interest is characterizing DNA, for example, the hybridization of DNA (e.g., to detect specific sequences of DNA), as well as other biological processes.
One method of DNA characterization, is to employ what is known as a gene chip comprising an array of known DNA molecules, and introducing the chip to a prepared sample. The sample is prepared by “tagging” the individual unknown DNA molecules with fluorescent dyes. When the sample is introduced to the gene chip, if the DNA hybridizes, the DNA can be identified using an optical microscope to determine which of the known DNA molecules was “tagged.”
This indirect measurement of DNA hybridization, although time efficient once the sample is prepared, is an expensive and very labor intensive process primarily due to the aforementioned sample preparation. Moreover, requiring an optical microscope has significant disadvantages, such as non-ideal resolution. In particular, each pixel on the gene chip has to be large enough to be seen by the optical microscope so it must be, typically, many microns wide.
Notably, the actual implementation of this method must include facilitating a reaction called polymerized chain reaction (PCR) to amplify the DNA. In this way, a small amount of DNA can make millions of copies of itself that can be fluorescently labeled. As a result, unfortunately, single molecules of DNA cannot readily be measured and imaged with the gene chip. In addition to this drawback, using the PCR technique is impossible for some types of samples. For example, there is no equivalent reaction to PCR for proteins. To detect some small amount of a protein, there is, as a result, a much smaller detection window.
In sum, in both SECM and STM, a significant drawback is that each system requires current flow between the tip and sample, as SECM uses electrochemical current and STM uses tunneling current as their respective feedback signal. As a result, the metrology field was in need of a system capable of monitoring electrochemical changes using a scanning probe microscope on an atomic scale, and without requiring continuous current flow. In addition, and among other applications, an apparatus capable of imaging single biological molecules, preferably without requiring extensive preparation of the sample was also needed.